Imagine an internal dialogue by one of the greatest minds in the Western tradition about the role of the Blessed Virgin Mary in salvation. Now place yourself in the middle of this conversation. This situation is possible for the first and last time at the exhibit “Leonardo da Vinci: Painter at the Court of Milan” at London’s National Gallery of Art.

One voice in the dialogue is that of the 30-year-old Leonardo, newly arrived from Florence, aspiring to be the court artist to Duke Ludovico il Moro, and awarded his first major commission in Milan in 1483.

The second take is from the 54-year-old Leonardo, painting a second altarpiece on the same theme in 1506. The world-famous artist already had completed his masterwork “The Last Supper” mural in the monastic dining hall of Santa Maria delle Grazie in Milan.

Both paintings are popularly called “The Virgin of the Rocks.” The first has been in France since Leonardo died there in 1519. The second was purchased by a Scottish nobleman in 1781 and ended up in the London National Gallery.

During the exhibit these two large paintings face each other across a gap of about 50 feet. Although they share many features, the altarpieces differ in crucial ways.

The earlier Paris version, painted while Leonardo regarded his role as a “mirror of nature,” lovingly evokes the natural world of plants, water, caverns and distant light. The four figures — the human Madonna and St. John, and the divine Christ and angel — form a circle that we contemplate across a pool of water in the foreground.

Related Film

The exhibit runs until Feb. 5, but for the first time ever, a 100-minute, high-definition look at the show will tour to selected movie theaters worldwide via satellite after Feb. 16, in “Leonardo Live.” A trailer of the film can be seen at bitly.com/leolive.

In the later London version, the figures are larger, posed in a subtly more frontal, theatrical arrangement. Their heads are more idealized, and the composition is more unified by light. Even the plants are rendered more abstractly. Until a recent cleaning verified that the London painting was entirely by Leonardo, many scholars thought that it was at least partly by students.

The exhibit features many of the world’s dozen or so authentic Leonardo paintings (including the unfinished “St. Jerome” from the Vatican, the portrait of Cecilia Gallerani in Cracow, Madonna “Litta” from St. Petersburg and the “Madonna of the Yarnwinder” from Scotland), a selection of Vincian drawings from the Milan period that permits a look into the intimate workings of his mind.

Art for the liturgy

The London show re-focuses Leonardo’s relation to the Catholic faith. As curator Luke Syson writes, “There has always been a tendency to over-secularize Leonardo; we should remember that over half of his paintings represent the prime protagonists of the Christian story: Christ, his Mother, and the saints. Leonardo, whose Christian beliefs were basically orthodox, would not have quarrelled with the idea that these holy men and women were the most miraculous of all the creations of his ‘prime mover.’”

As Leonardo evolved in Milan, he came to see the painter’s role not merely as a mirror but as godlike. “The divinity which is the science of painting transmutes the painter’s mind into the divine mind,” he wrote in a passage quoted by Syson. In one of his anatomical notebooks Leonardo argued passionately against those Churchmen who sought to limit his research into the human body “that the love of anything is the offspring of this knowledge, the love being more fervent in proportion as the knowledge is more certain.”

The famous pen study of St. James reacting to Jesus in the “Last Supper,” with the shock expressed by the bulging neck tendon and shifting definition of the eyes, vindicates the artist’s view that scientific observation is vital to expressing a sacred drama. When Christ says, “This is my body,” the apostle reacts to the coming Passion and Redemption with a wordless mixture of love and horror; Leonardo’s ability to depict this duality, barely visible in the much damaged mural today, rests on his anatomical investigations.

Art historian Timothy Verdon explains that the Dominican friars would return from Mass in the adjoining church, after listening to the Gospel and receiving the Eucharist, and then look up at the two great murals on either end of their refectory, Leonardo’s “Last Supper” and the “Crucifixion” by Donato di Montorsolo, both painted in the 1490s. The two pictures together expressed the meaning of the Passion of Christ and gave value to the sacramental life led by the friars.

Two ‘Virgins of the Rocks’

Why Leonardo, who hated to repeat himself, made two nearly identical editions of the “Virgin of the Rocks” remains puzzling. The London version was on the altar of the Chapel of the Immaculate Conception in San Francesco Maggiore in Milan from 1508 until 1781.

After a devastating earthquake and decades of neglect, the church was demolished in 1813, leaving us clueless about how Leonardo’s composition fit into its setting. We know that the chapel stood over the catacombs where early Christians were buried, and this may be a reason why Leonardo set the sacred story in a grotto.

Most scholars believe the Paris version, from the 1480s, was Leonardo’s first painting for that chapel. One explanation is that he withdrew the picture after a dispute over payments and had to paint a second one in 1506 to fulfill his contract.

Both paintings expressed the doctrine that Mary’s Immaculate Conception was planned by God from the beginning of time. The unique composition of the Virgin kneeling in a grotto, a kind of pre-Eden primeval Paradise, with the infant St. John Baptist, the blessing Christ Child and an angel, is a complete departure from the standard images of the Immaculate based on the Woman of the Apocalypse with her head encircled by stars — but then, Leonardo was nothing if not inventive when it came to religious subjects.

The elite Brotherhood of the Immaculate Conception commissioned Leonardo and two local painters to complete the elaborate altar.

Dispute over doctrine

The two great mendicant orders founded by St. Dominic and St. Francis had rescued the Church during the social upheavals of the 13th century. Both were devoted to the Blessed Virgin Mary, but their shared purpose did not mean they saw eye to eye.

Following Thomas Aquinas, the Dominicans held that Mary — and St. John the Baptist — was born without sin after being sanctified in her mother’s womb. The Franciscans reasoned that as Mother of God, Mary must necessarily be superior to John, and that she was free from sin from the moment of conception. (Centuries later, in 1854, the Church adopted the Franciscan view in promulgating the dogma of the Immaculate Conception.)

In Leonardo’s Paris altarpiece, the angel Gabriel who accompanies Jesus points toward the little St. John, the forerunner who predicted Christ’s passion. In a musical orchestration of the octave between the blessing hand of Jesus and the sheltering hand of Mary, this pointing finger strikes the interval of the fifth — a harmony musicians of that time, including Leonardo himself, believed to be divine. By placing Mary at the apex of the triangle and pointing to John, Leonardo underscored the Virgin’s clear superiority in keeping with Franciscan teaching.

Remarkably, X-rays of the London picture reveal that the artist originally envisioned a completely new composition, with the Virgin kneeling to one side adoring the infant Christ. Apparently he painted over this idea because the patrons wanted a closer replica of his first picture.